The present exemplary embodiment relates to a system and method to improve pitch stability for a vertical takeoff and landing aircraft. It finds particular application in conjunction with aircraft including side by side coaxial rotors, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Coaxial vertical takeoff and landing (VTOL) aircraft include rotors having a top propeller and a bottom propeller spaced from each other along a common axis of rotation. Conventional coaxial rotor aircraft systems are required to operate with a balance of torque. Torque balance is achieved by rotating the top propeller in an opposite direction from the bottom propeller with a common angular rate such that the combined rotation results in a torque profile equilibrium. The rotor systems of existing assemblies typically include an axially spaced distance between the top propeller and the bottom propeller of approximately 10% of the propeller diameter. The top propeller and bottom propeller are of equal length. This distance provides adequate space for flapping and bending of propeller blades and to assure adequate clearance between each rotor while the aircraft maneuvers. Fixed pitch propeller blades have been used but require optimization of the axially spaced distance between the propellers and its geometry.
Propellers in a side by side rotor system are dependent on geometric constraints of design. More particularly, the length of each propeller is a function of the space between each rotor and the size of the aircraft body. Rotating propellers must avoid contact with the aircraft body and with each other. Restricting blade size requires propellers to operate at increased rotational speeds to improve performance. However, high rotational speeds are not conducive to cyclic control to both maneuver the aircraft and improve pitch stability.
Attempts to control pitch stability in side by side VTOL aircraft include utilizing control surfaces in the propeller slipstreams and to provide longitudinal tilting of each rotor in relation to the aircraft. However, these solutions result in minimal effectiveness in part, due to the center of gravity and length of each arm from the propeller to the aircraft. Additionally, the longitudinal tilting of the rotors produces an adverse effect due to the generation of additional moment forces on the aircraft body.
Oblique active tilting (OAT) was introduced to remedy pitch stability control for VTOL aircraft with single propeller side by side rotors. OAT generates gyroscopic control moments that improve pitch stability independent from the propeller distance to the center of gravity. This results in an ability to reduce aircraft size while maintaining the same payload limits without losing control effectiveness. However, side by side VTOL aircraft with single propeller rotors have a limited lifting capability.
Therefore, there remains a need to provide a system and method to improve the lifting capacity of a compact VTOL aircraft with side by side rotors.